1. Field of the Invention:
The present invention relates to scanning radiometers and electro-optical systems. More specifically, the present invention relates to methods and apparatus for controlling the scan rate of scanning radiometers and electro-optical systems.
While the present invention is described herein with reference to illustrative embodiments for a particular application, it is understood that the invention is not limited thereto. Those of ordinary skill in the art having access to the teachings provided herein will recognize additional modifications, applications and embodiments within the scope thereof.
2. Description of the Related Art:
Radiometric resolution provides one measure of the performance of a scanning system. Radiometric resolution is defined as the amount of radiation that must be received by the system to produce an electrical signal equal to the system electrical noise. Wide-angle scanning systems will generally have a varying radiometric resolution over a scan line if the system does not have a surface scan velocity that provides a constant dwell time of the projected field of view at all scan angles. This variation in radiometric resolution generally imposes a significant constraint on the system in that the system must be designed so that its worse-case radiometric resolution meets the specified requirement. For example, a wide-angle scanning system scanning a typical range of plus or minus 56.25 degrees could require an increase in the aperture size by a factor of approximately 1.4 or an increase in the number of detectors used by a factor of approximately 2 if the surface scan velocity over the scan line is not designed to provide the optimum projected field of view dwell time. Such an increase in either the aperture size or the number of detectors would impose significant power, size, weight and cost requirements on the system.
Three well known methods of scan drive operation are: (1) constant angular rotation; (2) sinusoidal angular scan; and (3) constant surface velocity scan. Constant angular rotation systems, whether of the continuously rotating or the end-of-scan turnaround variety, provide optimum scan velocity only for systems that use a constant angular field of view over the scan line. Systems employing this technique are generally not as efficient as systems employing either of the other two techniques and offer poorer performance with respect to radiometric resolution (NEDT) and signal-to-noise ratio (S/N). (The footprint is the projection of the detector field of view onto the scan surface at an instantaneous scan angle.) In addition, systems employing this technique are not really adaptable to provide constant ground sample spacing of radiometric data over large scan angles for systems requiring this performance characteristic.
The sinusoidal angular scan drive technique typically provides oscillatory motion of a pivotally mounted dectector mechanism. This system is often utilized because it sustains the oscillatory scanning motion while requiring only the input of enough additional energy to overcome inherent frictional losses. The line-of-sight scan velocity of such systems is not constant, however. As a result, although sinusoidal angular scan drive systems are generally more efficient than constant angular rotation systems, the sinusoidal scan motion does not provide an optimum scan velocity for either the constant angular field of view of the constant footprint system designs.
Conventional constant surface velocity scanning systems utilize power drive mechanisms which input power to the scan drive system to continuously change the angular scan rate thereof in order to achieve a constant surface velocity scan for the projected field of view. Unfortunately, while generally offering superior performance, the power, cost, and weight associated with the power drive mechanisms of conventional constant surface velocity scanning systems, impose significant limitations on such systems.
Thus, there is a need in the art for an improved method and apparatus for providing constant or predetermined surface velocity for the projected field of view of scanning instruments which does not impose significant power and weight constraints on the system while offering constant radiometric resolution. There is a further need in the art for an apparatus which would provide a predetermined velocity profile of the projected field of view on a scanned surface.